It is known that lignocellulose biomass material can be transformed through thermo condensation (torrefaction) to a product that is well suited for the production of thermal energy. A process for torrefaction is disclosed in, for example, U.S. Pat. No. 4,787,917. During torrefaction, lignocellulose material such as wood is subjected to a roasting process that takes place at elevated temperature. According to a presentation by J. P. Bourgeois and Jacqueline Douat entitled “Torrefied wood from temperate and tropical species, advantages and prospects” and held in Göteborg, Sweden at “Bioenergy 84” 15-21 Jun. 1984, torrefaction is performed in a temperature range of 180° C.-400° C. Torrefaction can also be understood as “thermo condensation”, U.S. Pat. No. 4,816,572, which is here defined as isothermal treatment of lignocellulose materials to eliminate water and carbon oxides by chemical reactions.
The practical torrefaction process typically involves cutting or crushing a lignocellulose material into small pieces which are transported to a roaster where the material is exposed to elevated temperature in an inert atmosphere (oxygen free or low oxygen conditions). The temperature may be in the range of 180° C.-400° C. and typical values lie within a range of 250° C.-280° C. The roaster is a closed compartment which holds the inert gas phase, or a non-oxidizing inert gas may be passed through the roaster. The time used for torrefaction in the roaster depends largely on the temperature. U.S. Pat. No. 4,787,917 suggests that the process may be completed within 10 minutes and that 3-6 minutes may be considered suitable. Another document that discusses torrefaction, U.S. Patent Application Publication U.S. 2008/0022595, suggests that torrefaction is carried out at a temperature in the range of 250°-320° C. and with a residence time in the range of 10 to 40 minutes. During torrefaction, substances such as water vapor, hydrocarbons and carbon oxides leave the lignocellulose material and the residue is a solid product which has high energy content in relation to its mass. This improves the transport economy of the final product, i.e. it becomes cheaper to transport the torrefied product since its energy density is increased. The torrefied product is very friable and can thus easily be ground to particles that can be compacted to further increase the energy density.
Torrefied material has been subjected to elevated temperature during roasting, so it is very hot when it leaves the roaster and it is completely dry or the water content has been reduced to levels that are negligible. It is reactive and may very easily ignite in contact with airborne oxygen when it exits the reactor (the roaster). The torrefied material needs to be cooled to avoid the fire hazard. U.S. Pat. No. 4,787,917 suggests that torrefied material is cooled with a cold and non-oxidizing gas. The use of air would be inappropriate since it contains oxygen. U.S. Pat. No. 4,816,572 suggests that methods that are known per se may be used to control the atmosphere during the cooling process. The document mentions as an example the spraying of a small amount of water on the hot material to create a steam atmosphere and to increase the heat transfer rate at cooling, i.e. evaporative cooling. U.S. Pat. No. 4,816,572 further suggests that the material should not be contacted with an oxidizing medium until it has cooled below 200° C. Applying cooling water sprays on a bed of torrefied material is problematic since it may lead to an uneven cooling. Parts of the torrefied material bed may become sufficiently cooled while other parts remain at a temperature level which is dangerous when the material is later exposed to air. Adding more spray water will entail the disadvantage that some parts of the torrefied material receive too much water and becomes soaked. High moisture of the torrefied product must be avoided since it decreases the net heating value of the product. It is possible to even out the temperature differences in a bed of torrefied material by mixing it with a suitable device.
In this context, it is to be noted that torrefied material has a low thermal conductivity. The result is that the internal part of each individual particle in the bed of torrefied material is hotter than the surface of it. The bigger the particle, the hotter are the internal parts of the particles. The conclusion must be that cooling of a torrefied wood piece is a relatively slow process and that there is a risk that material pieces leave the cooling step with un-sufficiently cooled internal parts. There is a risk of exothermic reactions and fire when such a particle is penetrated by air.
It is desirable to compact the torrefied material to increase the density (e.g. combustible fuel pellets with high density) for reduction of the product volume at transportation and storage. The compaction machines typically need a feed temperature up to 100° C. in order to ensure a good pellet quality. The temperature of the torrefied material is typically around 250° C. when it exits the roaster. Such hot feed material will result in overheating of lubricants for bearings and rolls which results in wear and mechanical damage of the pelletizing machine. It is necessary to cool the torrefied product prior to pelletizing for protection of machinery. Reheating of torrefied material which is cooled to a very low temperature (much below 100° C.) prior to pelletizing will cause additional operation costs. The flow of cooling water should be controlled accurately from this point of view. Even when the quantity of water is very carefully selected, it may still be the case that parts of the torrefied material become soaked while other parts do not get sufficient cooling.
The biomass feed to torrefaction can be crushed or chipped forest residue. The particle size distribution of such feed is not directly suitable for a pelletizing process. Typical industrial pellets are made with dies having 6 or 8 mm holes through which the biomass is pressed to form pellets. The average particle size suitable for pelletizing should be below 3 mm (˜⅛ inch), so it is necessary to grind the torrefied material in a mill prior to pelletizing it to fuel pellets.
The materials handling (conveying) from the roaster to a mill entails several problems since the hot torrefied material is dusty and highly reactive (i.e. flammable, possibly even explosive under certain circumstances). It is important to prevent leakage of air to the material in any transporting solution from the roasting to the mill, even when the torrefied material has undergone a cooling procedure because the cooling may have failed to cool all parts of the torrefied material to a safe level. In principle, the conveying system and the mill can be made inert by means of steam or nitrogen for example. However, in practice it is difficult to make the system perfectly sealed and any frictional heat by moving parts increases fire hazard.
One objective of the present invention is therefore to provide a method and an arrangement that achieves a uniform cooling through the entire volume of the torrefied material before it is exposed to an oxygen containing environment.
Another objective of the invention is to ensure safe grinding conditions of torrefied material to particles with a correct temperature and size for pelletizing purposes.
A third objective of the invention is to provide a method and arrangement which permits safe torrefied material handling (conveying) from grinding to pelletizing.
These and other objectives of the invention are achieved by the present invention as will be explained in the following.